If the test of paired utterances shows them to be not linguistically equivalent, we still do not know exactly what the differences between the two utterances are. We cannot assume that the two utterances are pairs, i.e. will later turn out to be phonemically different in only one segment: If we compare 'Marx sat' with 'Mark’s sad' we could find a regular difference which will turn out to be phonemic in only segment [t]—[d] (although a difference also occurs between [æ·] and [æ]). But if we compare 'He sat' with 'He said' the differences will later appear to be phonemic in two segments: [æ]—[e·] and [t]—[d].
- Harris (1951), a pag.38